Carbohydrate-based chemical processes are of growing importance in view of the desire to use renewable feedstocks such as biomass. The isomerization of sugars is an important class of reactions used in various industrial carbohydrate-based processes. The conversion of glucose into fructose is one such process of particular significance. This reaction has been used for the production of high-fructose corn syrups (HFCS) as well as for the production of valuable chemical intermediates, such as 5-hydroxymethylfurfural (HMF) and levulinic acid.
The isomerization of glucose to fructose can be performed under mild conditions using either biological or chemical catalysts. This reaction is slightly endothermic (ΔH=3 kJ/mol) and reversible (Keq˜1 at 298 K), which means that the maximum attainable degree of conversion of glucose to fructose is governed by the thermodynamic equilibrium between both sugars at the reaction temperature.
Industrial glucose isomerization is generally accomplished using an immobilized enzyme, which offers the benefit of high conversion and selectivity, but also presents numerous challenges. The enzymatic catalysts do not maintain high activity over multiple cycles, cannot be easily regenerated, and do not perform over a wide variety of temperatures, pHs, salt concentrations, and other process conditions. Furthermore, enzymatic isomerization catalysts cannot be easily integrated into upstream processes for forming glucose from biomass, or into downstream processes for transforming fructose into other chemical intermediates.
For example, one preferred industrial isomerization method involves the use of an immobilized enzyme (xylose isomerase) at 333 K that generates an equilibrium mixture of 42% (wt/wt) fructose, 50% (wt/wt) glucose, and 8% (wt/wt) other saccharides. Although fructose yields are high, this enzymatic process has various drawbacks that include: (i) the need for various prereaction purification processes to remove impurities from the feed that strongly inhibit enzyme activity, e.g., calcium ions present from a previous starch liquefaction/saccharification step must be removed to levels <1 ppm, (ii) the use of buffered solutions to maintain an optimal pH between 7.0 and 8.0 (Na2CO3) and to activate the enzyme (MgSO4) that requires postreaction ion-exchange procedures, (iii) an optimal operating temperature of 333 K to maximize both product yield and enzyme lifetime that precludes faster reaction rates that could be attained at more elevated temperatures, and (iv) higher operating costs resulting from the periodic replacement of the catalyst bed due to the irreversible decay in activity suffered by the enzyme over time.
Accordingly, it is desirable to provide a process for isomerizing sugars with high conversion and selectivity without the drawbacks associated with enzymatic catalysts.